Diseases & Conditions

ACL injury

An ACL injury is the tearing of the anterior cruciate (KROO-she-ate) ligament in your knee. An ACL injury may make your knee feel unstable or loose, and your knee may "give way" if you return to your sport too quickly.

Although an active lifestyle benefits your overall health, exercise isn't always easy on your knees. The anterior cruciate ligament is especially susceptible to the demands of certain sports, such as volleyball, gymnastics, basketball, soccer and football.

Treatment of an ACL injury may include surgery to replace the torn ligament, along with an intense rehabilitation program. As for prevention, if your favorite sport involves pivoting or jumping, a proper training program can help you avoid an ACL injury.

At the time of an ACL injury, signs and symptoms may include:

• A loud "pop" sound
• Severe pain
• Knee swelling that usually worsens for hours after the injury occurs
• A feeling of instability or "giving way" with weight bearing

Once the swelling subsides, your knee may still feel unstable. It may feel as if it's going to "give way" during twisting or pivoting movements.

When to see a doctor
If you experience any of the signs and symptoms of ACL injury — a popping sound, severe knee pain, a swollen knee or a feeling that your knee is giving out — see a doctor. Also see your doctor if your knee feels loose or unable to support your weight. In general, the longer you wait to start treatment, the longer it will take to get better.

Ligaments are strong bands of tissue that attach one bone to another. The ACL, one of two ligaments that cross in the middle of the knee, connects your thighbone (femur) to your shinbone (tibia) and helps stabilize your knee joint.

Most ACL injuries happen during sports and fitness activities. The ligament may tear when you slow down suddenly to change direction or pivot with your foot firmly planted, twisting or overextending your knee.

Sports that involve running, turning sharply, pivoting and jumping — especially basketball, soccer and gymnastics — put your knee at risk. The ACL can also tear when the tibia is pushed forward below the femur, such as during a fall in downhill skiing. A football tackle or motor vehicle accident also can cause an ACL injury. However, most ACL injuries occur without such contact.

To diagnose a torn ACL, your doctor first wants to know as much as possible about the injury, such as whether you heard or felt your knee pop, whether your knee swelled up afterward and if you were able to continue being physically active.

Swelling that occurs shortly after the injury usually means there's blood in the joint from torn blood vessels in the damaged ligament. Your doctor may decide to draw the blood out with a needle and syringe. This can reduce pain and make it easier to examine the knee joint.

Your doctor examines your knee in a variety of positions to assess whether or not your ACL is torn. Two common exams are:

• Lachman's test. In this test you lie on your back on the exam table with your injured leg bent at a 30-degree angle and your foot flat on the table. Your doctor then moves the lower portion of your injured leg forward from the knee. If your leg moves freely without reaching a firm endpoint, you have a tear in your ACL.
• Pivot shift test. For this test, your injured leg is extended, and your doctor rotates your foot at the same time he or she applies pressure to the outside of your knee and bends your knee. Signs of instability in your shinbone suggest an ACL tear.

Often the diagnosis can be made on the basis of the physical exam alone, but you may need X-rays to rule out a bone fracture. If your doctor has questions about the cause or extent of your injury, he or she may order a magnetic resonance imaging (MRI) scan, a painless procedure that uses magnetic fields to create an image of the soft tissues of your body. An MRI can show the extent of ACL injury and whether other knee ligaments or joint cartilage also are injured.